Robot vacuum cleaners of the type referred to above are known; see for instance WO 9740734 and EP-A-803224. These robot vacuum cleaners, which, preferably, are battery driven, are provided with a circular housing and with means for sensing surrounding objects or obstacles so as to avoid, or otherwise deal with, such objects and obstacles during a vacuum cleaning operation. The vacuum cleaner is automatically guided past the objects or obstacles and can vacuum hard as well as soft floor surfaces. The driving wheels are typically arranged for rotation on separate horizontal shafts that are placed in coaxial alignment with one another for rotation about a common axis. Also, normally, the driving wheels are rotatably supported by bearings that are permanently fixed in relation to the housing. By means of the circular housing shape, and by driving the wheels at varying velocities and in different rotational directions, the vacuum cleaner can be automatically moved and guided such that any tendencies for the cleaner to become stuck or otherwise restrained in its operation are minimized.
Although the prior art arrangement described above works well under most circumstances, the fixed-wheel design with which the prior art vacuum cleaners are provided can result in operational failures when the vacuum cleaner encounters obstacles such as, for example, rugs having high or loose edges or thresholds. It is not always possible for such fixed-wheel cleaners to be guided past such obstacles. In order to minimize this difficulty, there are broad suggestions in the prior art, e.g. see U.S. Pat. Nos. 5,720,077 and 5,815,880, that a suspension mechanism can be provided for the driving wheels so as to allow the wheels to engage the floor surface even if there are recesses, undulations or the like in the floor surface. However, no specific wheel assembly is described for accomplishing that result.